The disclosure relates generally to welding furnaces, and more particularly to welding furnaces allowing special handling of work pieces made from superalloys.
Components of apparatus that are exposed to high temperatures and/or high stress environments, such as rotor blades in a gas turbine engine, for example, are generally made of high performance alloys. In particular, so-called “superalloys” have come into wide use for such applications. These superalloys typically are based on nickel and/or cobalt and are generally used to form articles by casting. However, circumstances arise in which components must be welded. For example, because superalloys are expensive, it has become desirable to repair or restore parts made from superalloys rather than outright replace such parts. While articles of relatively simple geometries may be welded in currently-available welding apparatus with success, articles of more complex geometries may not enjoy as successful or high quality welding in such apparatus, particularly where such articles are processed at elevated temperatures. In addition, superalloy parts tend to oxidize in ambient atmosphere at such elevated temperatures, which contributes to failure or poor quality of welds.
Some superalloy components require thermal stress relief prior to welding so that residual stresses in the components may be removed or relieved. For example, if a component has been removed from service in a gas turbine, such as for reconditioning or repair, stresses may remain in the component as a result of its service. Welding and/or cooling after welding may also induce stresses in components requiring additional stress relief, such as with heat treatment. As is known, heat treatment follows specific processes including ramping rates, soak temperatures, hold times, and cooling rates that improve the likelihood of achieving desired qualities in the final article or component. Deviation from these processes may result in flaws or undesired characteristics of the final article or component. As a result, it is recognized that a high degree of control over the environment in which an article is processed is desirable.
Heat treatment of articles in the past generally employed large-scale blast furnaces and the like in which large numbers of articles were treated at the same time. However, such large-scale solutions typically have long heat treatment times due to several factors. For example, a large blast furnace as a large mass to heat, and the typically large number of parts being heat treated in a batch adds additional mass. Additionally, long queuing times may occur while batches are assembled as individual components are repaired. Therefore, batch furnace pre-weld and post-weld stress relief heat treatments may cause delay in a welding line. Further, such large—scale blast furnaces typically have no control over the atmospheric composition to which the parts are exposed, thus exposing superalloy parts to potentially oxidizing ambient atmosphere.